What kind of corrosion is this?

[gray_sailor]

Hi!


This is a picture of the main boom on catamaran in Turkey.
We have a discussion about the type of corrosion and its nature.
Depending on the answer there are various reasons and remediation action for this corrosion.


Is it Pitting or Electrolytic?
What do you think?





Is anyone with appropriate competence able give us an opinion?
Thank you very much!



P.S. I vote for pitting

[double u]

"electrolytic": electric currents from outside are involved
"galvanic": dissimilar metals create a current

could this be caused by tiny particles of a nobler metal from some nearby discgrinding?

[gray_sailor]

Originally Posted by double u

"electrolytic": electric currents from outside are involved
"galvanic": dissimilar metals create a current

could this be caused by tiny particles of a nobler metal from some nearby discgrinding?

There are no other metals to start galvanic corrosion here.
This is an aluminum alloy mast boom and nothing else..

[s/v Jedi]

Originally Posted by gray_sailor

There are no other metals to start galvanic corrosion here.
This is an aluminum alloy mast boom and nothing else..

Still, I agree with double u… looks like dust/sparks from grinding steel got onto the boom. Grinding could have been somewhere else aboard or even a neighboring boat.

The only other possibility is a failure of the anodizing, which is normally caused by physical damage.

[flyingfin]

Buckshot

[double u]

doesn't look like anodized, btw...
another szenario: boom under the boat while copper-rich antifouling is rolled on. Minuscule droplets of AF are left on the boom & presto...
ever which way: now there are distinct galvanic cells (no oxygene at the bottom of the pits-less noble than aluminium higher up in the pit-galvanic corrosion concentrated at bottom of pits) that will eat through the thin walls pretty quickly. Probably best to at least "countersink" all pits, better even drill right through, remove all burrs

[Cadence]

Originally Posted by s/v Jedi

Still, I agree with double u… looks like dust/sparks from grinding steel got onto the boom. Grinding could have been somewhere else aboard or even a neighboring boat.

The only other possibility is a failure of the anodizing, which is normally caused by physical damage.

On the grinding, probably from another vessel, while on the hard.

[gray_sailor]

Originally Posted by s/v Jedi

Still, I agree with double u… looks like dust/sparks from grinding steel got onto the boom. Grinding could have been somewhere else aboard or even a neighboring boat.

The only other possibility is a failure of the anodizing, which is normally caused by physical damage.

Wow! Very interesting guess!
So, you don't give any chance neither for pitting nor for electrolytic corrosion here?

[gray_sailor]

Originally Posted by double u

doesn't look like anodized, btw...
another szenario: boom under the boat while copper-rich antifouling is rolled on. Minuscule droplets of AF are left on the boom & presto...
ever which way: now there are distinct galvanic cells (no oxygene at the bottom of the pits-less noble than aluminium higher up in the pit-galvanic corrosion concentrated at bottom of pits) that will eat through the thin walls pretty quickly. Probably best to at least "countersink" all pits, better even drill right through, remove all burrs

Yes, I don't see anodizing either. That's why pitting corrosion came to my mind. Isn't it more likely that other scenarios?

[s/v Jedi]

Okay, if not anodized then pitting is normal. It will be very hard to impossible to keep it looking good without anodizing. There are two options: just live with it, use some aluminum wheel polish now and then (Steve Dashew did this, said it lasted just as long) or paint it. For paint you may be able to use the new Awlgrip CF wash primer, followed by Awlgrip topcoat straight over that.

As the boom should be pretty new to look like this with bare aluminium, painting should be relatively easy.

[Clivevon]

Originally Posted by Cadence

On the grinding, probably from another vessel, while on the hard.

Me too.

[Lepke]

Pitting and crevice corrosion can also happen in aluminum. Once a pit starts, debris can be caught in the pit and speedup the corrosion.

[team karst]

'boom under the boat'; and 'grinding':

is this on both sides of the boom?

[jimbunyard]

How hard is it to look inside the boom?

I've seen this before in aluminum windsurfer masts; in that case it is corrosion from the inside out. Usually the mast is trashed because the corrosion is much more extensive inside.

Why it happens is more of a question, I always thought it was a combination of less than effective interior anodizing and trapped salt-laden water vapor.

[roverhi]

The boom probably is clear anodized or un-anodized with a clear paint coat. An un-anodized boom wouldn't have any sheen to it and would be a darker dull flat silver color. The corrosion is a breakdown of the coating, anodizing or paint, which could have a number of causes besides just age. Would think a 12 gauge would have left small dents. In any case wouldn't be concerned about servicability of the boom. Go sailing.